The clinical realisation of intensity modulated radiotherapy (IMRT) requires sophisticated technology and honed treatment procedures if high standards of quality assurance and time efficiency are to be met. The expense rises dramatically for highly irregular fluence distributions if multi-leaf collimators (MLC) are used. It has been shown in references 1 and 2 that the optimisation problem of IMRT allows sufficient latitude to fine-tune the resulting fluence distributions to increase clinical utility.
Although these methods facilitate the translation of fluence distributions into the motion patterns of the MLC, they do in general not ensure the most efficient application or even feasibility of the fluence distributions as delivered by the optimisation algorithm. Ideally, the limitations of the application device are already taken into account during the optimisation of the fluence distributions. This is of particular importance for application techniques where the beam is switched off while the leafs are moving to the positions which define the outline of the next field segment. In this case, the fluence distribution is piecewise constant with a finite, usually small number of discontinuities.
The necessity of piecewise constant fluences also arises in IMAT with the additional constraint that fluence steps may not be displaced relative to the beam between two positions of the arc by more than a certain distance dictated by the finite leaf velocity. A sliding window dMLC technique requires piecewise linear fluence profile with the additional condition that the increase or decrease between two vertices (which are the DICOM control points) is equal for all leaf rows, only the position of these vertices may vary between rows.